One of North America’s most beloved and familiar birds, the Yellow-rumped Warbler, may be at least three separate species, says a study to be published today in The Auk. If the species were to be split, it would upend a status quo that has lasted for almost five decades and would restore two cherished common names that many bird watchers still fondly use.
For most of the last century the Yellow-rumped Warbler was two species, the Myrtle Warbler of the East (and far north) and the Audubon’s Warbler of the West. But in 1973 scientists lumped them based on evidence that the two species routinely hybridize in a narrow zone in western Canada.
Now, evidence from more than 37,000 regions of the birds’ DNA suggests that Myrtle and Audubon’s really are separate species—and so is a third, isolated form known as “Goldman’s warbler” that is almost entirely restricted to Guatemala. A fourth form known as the “Black-fronted” warbler lives in the mountains of northern Mexico but its species status is more debatable, the study authors report.
The wealth of genetic data—far more than was available in 1973—provided enough resolution for the researchers to see clear patterns of natural selection, even though the overall degree of genetic difference was quite small. For instance, differences between Myrtle and Audubon’s forms were clustered within only about 60 relatively short regions of DNA. These hotspots likely contain the genes responsible for making Myrtle and Audubon’s warblers different, said lead author David Toews, who performed the research at University of British Columbia and is now a postdoctoral researcher at the Cornell Lab. Natural selection likely caused those genes to change while leaving the rest of the birds’ DNA relatively unaltered, he said.
Going farther, the team also determined where in the DNA the differences were located. That discovery opens the door to learning precisely what’s responsible for how the four forms look, sound, and act.
How Did We Get Four Forms?
It’s thought that the Myrtle and Audubon’s forms became separated over the last million years—a pattern common to other songbird species pairs such as Baltimore and Bullock’s orioles and Rose-breasted and Black-headed grosbeaks. Toews thinks that at some point, birds that became the Myrtle form moved eastward from northwestern North America and then became cut off from the other three forms by ice sheets.
So what about that hybrid zone that swayed scientists to lump the species back in 1973? It’s relatively narrow—just 80 miles across—and recent research shows it has not moved or widened in 50 years, Toews said. Although the two forms interbreed freely in that zone, there must be some sort of weakness in hybrids that keeps them from surviving and carrying the genetic mixing farther afield. Although what that weakness is, exactly, remains to be discovered, Toews said.
There will be no immediate change to birding lists: formally splitting the species requires a decision by the North American Classification Committee of the American Ornithologists’ Union. They publish an annual set of updates to the official checklist each year in July.
A quick guide to the four forms of Yellow-rumped Warbler
Affectionately known to bird watchers as “butterbutts,” Yellow-rumped Warblers are abundant migrants that flood through North America each spring and fall. These migratory forms (Myrtle and Audubon’s) occur in most of Canada and in every U.S. state except Hawaii. Breeding males are handsome gray-and-yellow birds; females and nonbreeding birds are browner rather than gray, with fainter yellow patches and indistinct streaking. The following descriptions of the four forms center on the distinctive males.
Myrtle is the form that birders in the eastern U.S. and most of Canada see. It’s the most different of the four forms, with a white instead of yellow throat, a longer migration, and a distribution that stretches farther north than the other forms. On migration they can be among the most numerous warblers seen during spring and fall migration—particularly along the U.S. East Coast, where they earn their name by feasting on energy-rich wax myrtle berries. They are unique among warblers in being able to digest the waxy fruits, and this allows them to winter much farther north than other warblers, sometimes as far north as Nova Scotia, according to Birds of North America. [update 2/8/22: Birds of North America is now part of Birds of the World.] Although many do winter in the southern and Mid-Atlantic U.S., others fly over the Gulf of Mexico to sites in the Caribbean and in Central America. According to the new study, this form is likely a separate species from Audubon’s and Goldman’s.
Audubon’s is the form familiar to bird watchers in the western U.S. and portions of Alberta and British Columbia. Males have yellow throats, large white flashes on the wings, and gray on the head, back, and breast. They don’t migrate as far as the Myrtle form, spending winters in parts of the western U.S. through Mexico to Honduras. The call note of Audubon’s is softer and less harsh than Myrtle. The two forms interbreed in a narrow zone in Canada at the northern edge of the Audubon form’s range. According to the study, the Audubon form is likely a separate species from Myrtle and Goldman’s.
Black-fronted: Whereas Myrtle and Audubon’s forms are fully migratory, Black-fronted (and Goldman’s) warblers are resident year-round in their range. The Black-fronted form occurs in the mountains of Chihuahua and Durango, Mexico. Males are darker gray above than Audubon’s, with black on the face, forehead, and chest. Evidence for considering this form as a full species vs. as a subspecies of Audubon’s is unclear—even the study authors themselves wrote that they could not agree on the issue based on the evidence in hand.
Goldman’s: This is the most isolated form of Yellow-rumped Warbler, occurring only in a small region of Guatemala (and possibly adjacent Chiapas, Mexico), fully 1,000 miles from the nearest breeding population of Black-fronted Warblers. (Both of the migratory forms, Audubon’s and Myrtle, can occur in Guatemala during winter.) Goldman’s is extensively black on the head, back, and breast, with a throat patch that is yellow bordered with white. According to the new study, this form is likely a separate species.
What’s your take? If the Yellow-rumped Warbler were to be split into Myrtle and Audubon’s (and Goldman’s), would you miss the yellow-rumped name or rejoice at the return of the old names? Would you still call them “butterbutts”? We’d love to hear what you think in the comments section.
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